Method of siliciding thermostructural composite materials, and parts obtained by the method

ABSTRACT

Within the pores of a porous thermostructural composite material, there is form an aerogel or xerogel made up of a precursor for a refractory material, the precursor is transformed by pyrolysis to obtain an aerogel or xerogel of refractory material, and then it is silicided by being impregnated with a molten silicon type phase. The aerogel or xerogel is formed by impregnating the composite material with a composition containing at least one organic, organometalloid, or organometallic compound in solution, followed by in situ gelling. The method is applicable to improving the tribological properties or the thermal conductivity of C/C or C/SiC composite material parts, or to making such parts leakproof.

CROSS REFERENCE TO RELATED PUBLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/545,734 filed Aug. 16, 2005 which is a §371 national phase filing ofPCT/FR2004/000345 filed Feb. 16, 2004, and claims priority to a Frenchapplication No. 0301871 filed Feb. 17, 2003.

BACKGROUND OF THE INVENTION

The invention relates to siliciding thermostructural compositematerials.

Thermostructural composite materials are known for their good mechanicalproperties and their ability to conserve those properties at hightemperature. Such materials are typically carbon/carbon (C/C) compositeshaving carbon fiber reinforcement and a carbon matrix, and ceramicmatrix composites (CMCs) with fiber reinforcement made of refractoryfibers (in particular carbon fibers or ceramic fibers) and a ceramicmatrix, or a matrix both of carbon and of ceramic (e.g. a matrix ofsilicon carbide SiC or a combined C/SiC matrix).

Parts made of C/C or CMC material are made by preparing a fiberstructure or “preform” of a shape close to that of the part to be made,and densifying the preform with the carbon or ceramic matrix.Densification can be performed by a liquid technique or by a gastechnique. The liquid technique consists in impregnating the preformwith a liquid composition containing a precursor for the matrix,typically a resin. The precursor is transformed by heat treatment,thereby pyrolyzing the resin. The gas technique consists in performingchemical vapor infiltration (CVI). The preform is placed in an oven intowhich a reaction gas is introduced. The pressure and temperatureconditions in the oven are adjusted so as to enable the gas to diffusewithin the fiber preform and form a deposit of matrix material on thefibers, either by one of the components of the gas decomposing, or elseby a reaction taking place between a plurality of components. Thosemethods of densification by a liquid technique or a gas technique arewell known in themselves, and they can be associated with each other.

Whatever the fabrication method used, thermostructural compositematerials present residual pores constituted by pores of greater orsmaller size (macropores and micropores) that communicate with oneanother.

Proposals have been made to finish off the densification ofthermostructural composite materials by siliciding, i.e. by introducingmolten silicon. The object is to modify the thermomechanicalcharacteristics of the materials, e.g. by increasing thermalconductivity or by making the materials more leakproof and/or reducingthe cost of final densification, since the conventional method employingthe liquid technique or the gas technique then does not need to becontinued for the time required to obtain the maximum density that ispossible by the method.

Depending on the nature of the composite material, siliciding may berectional or non-rectional. An example of rectional siliciding, asdescribed in particular in U.S. Pat. No. 4,275,095, consists in taking acomposite material having a matrix comprising carbon at least in anouter phase of the material, and in impregnating it with molten siliconthat then reacts with the carbon in order to form silicon carbide. Anexample of non-rectional siliciding is using molten silicon toimpregnate a composite material in which the matrix is made of siliconcarbide, at least in an outer phase of the matrix, i.e. a compositematerial in which the outer geometrical surface and the surfaces of thepores communicating with the outside are made of silicon carbide.

Molten silicon is very fluid and possesses high wetting ability,particularly on surfaces of carbon or silicon carbide. When athermostructural composite material is impregnated with silicon in theliquid state, the silicon advances into the array of pores in thematerial following the surfaces of the pores. As shown verydiagrammatically in FIG. 1, micropores and narrow passages orconstrictions in the material M are filled in, however macropores arenot filled in since the silicon (Si) flows along their surfaces. Theextent to which the pores are filled in is thus random, which means thatit is not possible to control thermal diffusivity and leakproofing. Inaddition, occluded gas pockets are formed that constitute inaccessibleclosed pores such as P.

Methods have been proposed for filling the pores of the compositematerial in part before performing infiltration with molten silicon.

Thus, document EP 0 835 853 proposes impregnating the material with anorganic resin and performing heat treatment to pyrolize the resin.Nevertheless, the grains of carbon (resin coke) that are obtained are tobe found not only in the macropores where they occupy part of theirvolume, but also in micropores or in constrictions in the array ofpores. Under such circumstances, while siliciding, the silicon reactswith the carbon of the grains, thereby increasing their volume andclosing off a pore, thereby preventing the silicon from passing. Thisresults in siliciding that is irregular. Furthermore, in particular inthe macropores, there remains a carbon phase that is sensitive tooxidation and that is constituted by the resin coke grains that have notbeen silicided or that have not been silicided sufficiently.

Proposals are also made in document U.S. Pat. No. 5,865,922 toimpregnate the thermostructural composite material with a resin having arelatively high coke content together with a pore-generating agent. Thisagent serves to form a foam prior to polymerization of the resin, sopyrolysis gives a carbon residue that is porous, and that issubsequently impregnated with silicon. That method likewise does notguarantee uniform filling of the initial pores in the composite materialby siliciding. While the foam is forming, the resin can flow back outfrom the material leading to a variable resin content in the material,and irregular porosity in the porous residue that results frompyrolyzing the resin. Furthermore, the transformation into foam canitself be irregular, with relatively large grains of carbon residuebeing formed that are not silicided in full, and with closed pores beingformed in the foam that remain inaccessible to the silicon.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to propose a method of treatingporous thermostructural composite material that makes it possible toimplement regular siliciding throughout the pores of the material, andto do so in a manner that is controllable and reproducible.

This object is achieved by a method of the type comprising impregnatingcomposite material with a phase of the molten silicon type, in whichmethod, according to the invention, prior to impregnation with a silicontype phase, an aerogel or xerogel of a precursor for a refractorymaterial is formed within the pores of the composite material, and theprecursor is transformed by pyrolysis in order to obtain an aerogel orxerogel of refractory material.

Advantageously, the aerogel or xerogel is formed by impregnating thecomposite material with a composition containing at least one organic,organometalloid, or organometallic compound in solution, and by gellingin situ.

In a particular implementation of the method, prior to impregnating witha silicon type phase, the steps of impregnation with a compositioncontaining at least one organic, organometalloid, or organometalliccompound in solution followed by in situ gelling are repeated aplurality of times.

In an implementation of the method of the invention, an aerogel or axerogel constituted by an organic material that is a precursor of carbonis formed within the pores of the composite material, which organicmaterial, after pyrolysis, gives a carbon aerogel or xerogel.

The organic aerogel or xerogel can be formed by impregnating thecomposite material with a composition containing an organic resinprecursor in solution.

Organic aerogels and xerogels suitable for providing carbon aerogels orxerogels after pyrolysis are well known. Reference can be made inparticular to U.S. Pat. No. 4,997,804 and to an article by L. Kocon inthe publication “Revue Scientifique et Technique de la Direction desApplications Militaires” [Scientific and Technical Journal of theMilitary Applications Directorate], No. 24, March 2001, pp. 30-140 andentitled “Céramiques Poreuses, Aérogels de silice et de carbonne”[Porous ceramics, silica and carbon aerogels], to an article by R.Preticevic et al. in the publication “Carbon”, 39 (2001) of “ElseviersScience Ltd”, pp. 857-867, entitled “Planar fiber reinforced carbonaerogels for application in PEM fuel cells”.

In another implementation of the method of the invention, an aerogel orxerogel constituted by a precursor of a ceramic type refractory materialis made in the pores of the composite material, and after pyrolysis thatgives a ceramic aerogel or xerogel. In the present specification, arefractory material is said to be of the “ceramic type” when therefractory material is other than carbon, in particular of the carbide,nitride, boride, or oxide type.

The aerogel or xerogel of ceramic precursor material can be formed inparticular by impregnating the composite material with a compositioncontaining an organosilicon compound in solution, for example anorganosilicon compound that is a precursor of silicon carbide, such aspolycarbosilane.

The above-identified article by L. Kocon describes how to make aerogelsout of oxide type refractory material.

The composite material is silicided after at least one refractorymaterial aerogel has been formed.

The term “siliciding” is used herein to mean impregnating thethermostructural composite material with a phase of the molten silicontype that penetrates into the pores of the composite material, the“phase of the silicon type” being constituted:

-   -   either by silicon and/or germanium (i.e. silicon on its own,        germanium on its own, or a mixture of silicon and germanium in        any proportions);    -   or else for the most part by silicon and/or germanium alloyed        with at least one metal or another metalloid.

Under such circumstances, the metal or other metalloid may be selectedin particular from iron, cobalt, titanium, zirconium, molybdenum,vanadium, carbon, or boron, depending on the particular properties thatare to be conferred on the thermostructural composite material aftersiliciding, or in order to prevent elements constituting the compositematerial matrix from dissolving in the silicon type phase.

An organic, organometalloid, or organometallic aerogel or xerogelpresents a gossamer structure which, after pyrolysis, gives a veryporous three-dimensional array of refractory material made of particlesthat are very fine. These particles are filamentary arrangements ofnanoparticles, i.e. of diameter of the order of about 10 nanometers(nm); that is a particular nanometric structure that gives the aerogelor xerogel a nanomaterial characteristic and that is quite differentfrom the structure of a foam as envisaged in U.S. Pat. No. 5,865,922.

The array formed by the aerogel or xerogel subdivides the initial poresof the composite material so that pores are obtained that are regular,in communication with one another, and without any particles beingformed that might lead to pores being obstructed, thereby impedingprogress of the silicon type phase. This leads to siliciding that isregular.

In addition, and this is a remarkable result that the inventionprovides, when the aerogel or xerogel is made of carbon, the fineness ofthe carbon particles, of nanometer size, means that they are silicidedin full, such that after siliciding, there does not remain anyoxidizable carbon phase derived from the carbon aerogel or xerogel, anda nanoarray of carbide particles is obtained that is dispersed in thesilicon type phase.

A method in accordance with the invention can also be used for bindingtogether parts made of thermostructural composite material. After therespective surfaces of the parts that are to be joined together havebeen brought side by side, a method of the kind defined above can beimplemented, comprising forming a aerogel or xerogel of refractorymaterial within the pores of the composite materials of the parts andwithin the interface or joint between said surfaces of the parts,followed by siliciding by impregnation with a silicon type phase.

The use of an aerogel or xerogel of refractory material, characteristicof the method in accordance with the invention, thus turns out to beparticularly advantageous for achieving regular densification bysiliciding, making it possible to obtain characteristics that areuniform and reproducible, in particular in terms of thermalconductivity, leakproofing, tribological properties, . . . .

Specifically when making thermostructural composite materials leakproof,siliciding can be followed by a step of forming a surface coating ofceramic material, e.g. by chemical vapor infiltration or deposition.

With a carbon aerogel or xerogel, siliciding involves reacting with theaerogel or xerogel, thereby transforming it into a nano-particulaterefractory material of ceramic type.

In contrast, with an aerogel or xerogel made of a ceramic typerefractory material, siliciding need not lead to reaction with theaerogel or xerogel.

In both cases, and according to another aspect of the invention, asilicided thermostructural material part is obtained in which thethermostructural composite material comprises a silicon type phasecontaining at least one aerogel or xerogel, i.e. a nanometric array ofceramic type refractory material.

In a particular application, the silicided thermostructural compositematerial part is a friction part comprising a carbon/carbon compositematerial with pores that are filled in at least in part by a siliconphase that contains a nanometric silicon carbide array. Such frictionparts made of silicided C/C composite material and in the form of diskscan then be used for providing a set of stator and rotor disks for amultidisk airplane brake.

In a variant, in a set of stator and rotor disks for an airplane brake,it is possible to associate C/C composite material rotor disks that havebeen silicided in accordance with the invention with C/C compositematerial stator disks that are not silicided, or vice versa.

In another particular application, silicided thermostructural compositematerial parts obtained by a method in accordance with the inventionconstitute electrodes, in particular anodes and/or cathodes and/oraccelerator grids, for ion or plasma engines, or indeed bipolar platesfor fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription given by way of non-limiting indication and with referenceto the accompanying drawings, in which:

FIG. 1, described above, shows very diagrammatically the result obtainedby performing siliciding on a thermostructural composite materialwithout subdividing its pores;

FIG. 2 is a flow chart showing the successive steps in an implementationof a method of the invention;

FIGS. 3 to 8 are diagrams showing different ways of impregnating athermostructural composite part with a molten silicon type phase;

FIGS. 9 and 10 are flow charts relating to variants of theimplementation of the method shown in FIG. 2;

FIG. 11 is a highly diagrammatic view of an application of a method inaccordance with the invention for using siliciding to bond togetherparts made of thermostructural composite material;

FIGS. 12 and 13 are photographic views taken using an opticalmicroscope, showing a nanometric array of ceramic particles that resultsfrom completely siliciding a carbon xerogel and a carbon aerogel usingsilicon and situated in a silicon type matrix, within the pores of athermostructural composite material; and

FIG. 14 is a photograph taken with an optical microscope showing asilicon phase in the pores of a thermostructural composite material, butwithout prior introduction of a xerogel or an aerogel into said pores.

DETAILED DESCRIPTION OF EMBODIMENTS

This application is a divisional of U.S. patent application Ser. No.10/545,734 filed Aug. 16, 2005 which is a §371 national phase filing ofPCT/FR2004/000345 filed Feb. 16, 2004, and claims priority to a Frenchapplication No. 0301871 filed Feb. 17, 2003 all of which areincorporated by reference herein.

FIG. 2 shows the successive steps in an implementation of a method inaccordance with the invention. The method is implemented on a piece ofthermostructural composite material, in particular a C/C composite or aCMC, having fiber reinforcement and a matrix densifying the fiberreinforcement. The thermostructural composite material presents poresmade up of pores of greater or smaller size that communicate with oneanother. Overall porosity is greater or smaller depending on the degreeto which the composite material has been densified.

The purpose of the method is to silicide the thermostructural compositematerial part so as to confer particular properties on the material,such as thermal conductivity, leakproofing, or tribologicalcharacteristics. The method also seeks to implement a final step ofdensifying the material under conditions that are less expensive thanthose that would otherwise need to be implemented in order to finish offdensification by a conventional liquid technique or by chemical vaporinfiltration.

An optional first step 10 in the method of FIG. 2 consists in performingtreatment to clean the accessible surfaces of the composite material,particularly when the material comprises a silicon carbide (SiC) matrixphase having a surface on which a film of oxide, in particular of silica(SiO₂), might have formed. It is desirable to eliminate the silica filmin order to encourage wetting of the surfaces of the pores duringsubsequent siliciding. To this end, it is possible to implement heattreatment causing SiO and CO gas to be formed by reaction between SiO₂and SiC. The heat treatment temperature can be lowered by performing thetreatment under low pressure. It is also possible to attack the oxidefilm of SiO₂ (or of SiOC) by means of a hot reagent gas such as carbondioxide CO₂ or sulfur hexafluoride SF₆. The accessible surfaces of thecomposite material can also be cleaned by acid attack, e.g. usinghydrofluoric acid HF and/or nitric acid HNO₃.

Thereafter, the thermostructural composite material is impregnated witha solution made up of components that serve, after gelling, ripening,and drying, to obtain an organic aerogel or xerogel (sol step 12).

By way of example, for the sol step, it is possible to use an aqueoussolution containing resorcinol and formaldehyde together with anoptional catalyst such as sodium carbonate. Various examples ofsolutions are given in U.S. Pat. No. 4,997,804. Other precursors oforganic gels in the hydroxybenzene family can be used such asphloroglucinol in solution in water and associated with a reactant suchas formaldehyde.

Gelling (polymerization) (gel step 14) is performed in situ by moderateheating, where the temperature must remain below the evaporationtemperature of the solvent. With an aqueous solution, this temperaturegenerally lies in the range 50° C. to 100° C. In the above example,gelling consists in performing a reaction between the resorcinol and theformaldehyde. Gelling is followed by a ripening step of duration thatmay cover one to several days, the material being left at the gellingtemperature.

This produces a three-dimensional (3D) gossamer array of athree-dimensional organic gel holding the solvent captive bycapillarity.

It should be observed that solvents other than water could be usedinsofar as they constitute solvents for the components of theimpregnation composition, but are not solvents of the polymer obtainedafter gelling.

Thereafter, the solvent is eliminated by drying under conditions thatensure that the 3D array does not collapse in spite of the capillaryforces acting on the filamentary components of the gel.

Drying may be performed by putting the gel into conditions that aresupercritical for the solvent (step 16) by increasing pressure and thentemperature so as to be situated beyond the critical point, and theneliminating the solvent in the hyperfluid state by isothermaldecompression, thereby achieving elimination without boiling. Aftercooling, a dry gel, or aerogel, is obtained in the form of a highlyporous 3D structure (step 18). Where appropriate, it is possible tobegin with a solvent exchange operation for replacing the solvent of theimpregnation composition with some other solvent that lends itself wellto supercritical drying.

In a variant, drying can be performed in controlled manner by slowevaporation (step 20). For example, after gelling and ripening in aconfined atmosphere, the composite material may simply be left in air toallow the solvent to evaporate. A dry gel or “xerogel” is then obtainedin the form of a porous 3D structure (step 22).

It should be observed that supercritical drying makes it possible toconfer on aerogels a structure that has few fissures in comparison withxerogels that may present fissures. Fissures are often caused by the gelshrinking within the pores, since the bonding between the gel and thepore wall is stronger than the breaking strength within the gel. Suchfissures can be advantageous while siliciding since they provide accesspaths into the volume of the xerogel for the molten silicon type phase.

The resulting aerogel or xerogel is pyrolyzed (step 24). This isperformed by raising its temperature to above about 600° C., e.g. to atemperature in the range 600° C. to 2000° C., or even higher, under anatmosphere of an inert gas such as nitrogen or argon, or under a vacuum.This produces an aerogel or xerogel made of carbon (step 26).

The following step 28 consists in siliciding. As mentioned above, theterm “siliciding” is used herein to mean impregnating the pores of thethermostructural composite material with molten silicon and/or germaniumalone or alloyed with at least one other element of the metal ormetalloid type, with the silicon and/or germanium nevertheless remainingin the majority.

For siliciding purposes, the composite material part is taken to atemperature lying in the range about 1400° C. to 1500° C., for example.Advantage can be taken of this rise in temperature to pyrolyze theorganic aerogel or xerogel (step 26 above).

The composite material can be impregnated with the silicon type phase invarious known manners. Several are described below for a silicon typephase that is constituted by silicon.

A first manner (FIG. 3) consists in placing the thermostructuralcomposite material part 30 that is to be impregnated and that has beenprovided with the carbon aerogel or xerogel on the surface of moltensilicon 32 contained in a crucible 34. The part is supported by studs36, e.g. of porous graphite, standing on the bottom of the crucible andfeeding the part with molten silicon by capillarity.

A second manner (FIG. 4) consists in placing the part 40 close to acrucible 44 containing molten silicon 42, and in using a drain 46 astransport means, the drain having one end immersed in the crucible andthe other end placed in contact with the part. The drain serves totransport silicon in the liquid state by capillarity from the crucibleto the part.

The drain may originally be formed by a mesh of continuous carbonfilaments or by a braided core, or by a braid of discontinuous carbonfilaments. Advantageously, it is also possible to use a wick, cord, orbraid made of SiC filaments or of carbon filaments pre-impregnated withpure or alloyed silicon.

The end in contact with the part may be secured thereto by a spot ofadhesive, by binding, or by pinching. The carbon filaments are silicidedand converted into silicon carbide by coming into contact with themolten silicon.

In a variant, and as shown diagrammatically in FIG. 5, the end of thedrain 46 that is in contact with the part can be received in a blindhole 47 formed in the part 40 so as to facilitate feeding the entirevolume of the part with the silicon type phase.

As shown in FIGS. 4 and 5, the part can be fed in the vicinity of oneend thereof, with the molten silicon progressing through the pores inthe composite material part containing a carbon aerogel or xerogel. Thepores, being subdivided by the aerogel or the xerogel, become filled inprogressively. When the accessible volume has been filled, excesssilicon remains in the crucible. Nevertheless, the quantity of siliconpresent in the composite material can be adjusted by adjusting thequantity of molten silicon that is delivered during siliciding.

Siliciding is rectional with respect to the aerogel or xerogel if it ismade of carbon, in which case it is transformed into an aerogel orxerogel of silicon carbide by reacting with the molten silicon.Siliciding may also be reactive with the thermostructural compositematerial depending on the nature of the matrix material forming thesurfaces of the pores through which the molten silicon travels.

After siliciding, a thermostructural composite material part is obtainedwith a silicon matrix phase containing an aerogel or xerogel made ofsilicon carbide.

It may be useful to control the flow of silicon at the surface of thecomposite material of the part 40, and more particularly to prevent itflowing so as to encourage the silicon to penetrate into the compositematerial on coming into contact with the part.

For this purpose, it is possible to deposit material around the contactarea between the drain and the part for the purpose of opposing any flowof silicon on the surface, i.e. a material that is not wetted by siliconand that does not react therewith. A material that is suitable for thispurpose is a material based on hexagonal boron nitride BN. As shown inFIG. 6, it can be implemented in the form of a bead 48 surrounding thecontact area between the drain 46 and the part 40, thereby preventingthe silicon from spreading over the surface of the part 40. The bead 48may be formed of BN paste of the kind sold under the reference “CombatBoron Nitride” by US supplier Carborundum.

Instead of the bead 48, or in combination therewith, it is possible tospray a BN film onto the surface of the part 40 other than in the areaof contact between the drain 46 and the part. The product sold under thename “DN60” by Acheson, a department of the British National Starch andChemical Company can be used for this purpose.

BN or some other material having the same function need not be depositedall the way round the area of contact between the drain and thecomposite material surface of the part 40, depending on the extent towhich it is desired to limit the flow of silicon on said surface.

In a variant, in particular when the thermostructural composite materialpart for siliciding is of relatively large size, it can be fed withmolten silicon via a plurality of points.

Thus, FIG. 7 shows a part 50 which is fed from two crucibles 54 ₁ and 54₂ containing molten silicon 52. The crucibles are connected to the partvia respective drains 56 ₁ and 56 ₂ which come into contact with thepart in the vicinity of opposite ends thereof.

FIG. 8 shows a part 60 that is fed from a single crucible 64 containingmolten silicon 62, by means of two drains 66 ₁ and 66 ₂ connecting thecrucible to two opposite end portions of the part.

As mentioned above, the impregnation method shown in FIGS. 3 to 8 can beused with a silicon type phase other than one constituted by siliconalone, i.e. a phase containing germanium and/or at least one otherelement of metalloid type or of metal type selected, for example, from:boron, carbon, iron, cobalt, titanium, zirconium, molybdenum, andvanadium. In any event, siliciding is preferably performed at atemperature that is only slightly higher than the melting temperature ofthe silicon type phase, e.g. within 15° C. or even 10° C. above saidmelting temperature. Under such conditions, the viscosity of the silicontype phase remains relatively high, thereby enabling the aerogel and thepores in the composite material to be invaded slowly, thereby ensuringthat filling is more complete.

Associating germanium with the silicon can serve to lower the meltingpoint in order to avoid possible degradation of the fiber reinforcementof the composite material when said reinforcement is made of fibers thatremain stable only up to a temperature close to or below the meltingpoint of silicon (about 1410° C.). This applies in particular to SiCfibers as sold under the name “NLM 202” by the Japanese supplier NipponCarbon for which stability can be affected at temperatures below 1250°C. The presence of germanium also makes it possible, by oxidation, toform germanium oxide GeO₂ which, like SiO₂, forms a glass that providesprotection against oxidation. Compared with SiO₂, an advantage of theoxide GeO₂ is that it forms at lower temperature and has a lowersoftening temperature. This makes it possible to extend the range overwhich there is provided an ability to self-heal the cracks that mightappear in the surface of the material, with self-healing being producedby softening of the vitreous compounds formed by oxidation. Healing thecracks protects the material from the surrounding oxidizing medium andcontributes to providing effective protection against oxidation.

Adding boron makes it possible firstly to lower the melting temperatureof the silicon type phase, and secondly to form a borosilicate typeglass (SiO₂, B₂O₃) by oxidation that presents good self-healingproperties and thus provides protection against oxidation. When thematrix of composite material contains boron, e.g. when it is a matrixcontaining at least one phase of the Si—B—C type as described in U.S.Pat. Nos. 5,246,736 and 5 965 266, the use of a silicon type phase thatis saturated with boron also makes it possible to avoid the boron of thematrix diffusing in the silicon type phase during the siliciding heattreatment which would lead to degradation of the composite materialmatrix.

Associating at least one metalloid other than Si or Ge and/or a metalcan make it possible to ensure that no free silicon (or germanium)remains after siliciding, thereby imparting an improved refractorynature to the material. For example, it is possible to use a silicontype phase containing an alloy of silicon and molybdenum in which thequantity of silicon is selected so that after siliciding, all of thesilicon has reacted to form SiC by reacting with the carbon of theaerogel or xerogel and by forming MoSi₂, which is a highly refractorycompound, by reacting with the molybdenum. The same applies when all orsome of the molybdenum is replaced by some other metal from thosementioned above.

Associating carbon and titanium with the silicon can produce a compoundTi₃SiC₂ which is a strong refractory ceramic of lamellar structure. Bydissipating energy between layers, such a lamellar structure makes itpossible to stop cracks propagating within the composite material,thereby improving its strength against impacts.

The carbon content of the final aerogel or xerogel can be controlled intwo manners. A first manner consists in controlling the reaction betweenthe components in the aqueous solution used for impregnating thethermostructural composite material, e.g. the ratio between theresorcinol, the formaldehyde, and the sodium carbonate. The secondmanner consists in repeating the introduction of an aerogel or xerogel.Thus, FIG. 9 shows another implementation of the method which differsfrom that of FIG. 2 in that after step 18 of obtaining an aerogel orstep 22 of obtaining a xerogel, there are performed steps 70, 72, 74 or70, 72, 78 of impregnating with a solution containing a precursor of anorganic cell (step 70), of gelling and ripening (step 72), and of dryingsupercritically (step 74) or by evaporation (step 78). This produces asecond aerogel structure (step 76) or xerogel structure (step 79)interpenetrating with the first.

The pyrolysis step 24 serves to obtain a carbon aerogel or xerogel (step26) that is more dense than that which is obtained by the method of FIG.2, while still remaining nanometric and gossamer-like. Siliciding isthen performed (step 28).

Forming an aerogel or xerogel prior to siliciding can be repeated morethan twice.

In a variant, it is possible to form in succession an aerogel and then axerogel (or vice versa). The probability of the xerogel fissuring,providing access paths to the silicon type phase during siliciding, canthen be of interest insofar as the prior filling of the pores in thecomposite material is more dense.

It should be observed that increasing in the quantity of carbon byforming a plurality of aerogels and/or xerogels in succession enables alarger fraction of the silicon (and/or germanium) introduced for thereaction with the carbon to be consumed, and thus serves to reduce thequantity of free silicon (and/or germanium) that remains in the end inthe material.

In yet another implementation of the method of the invention (FIG. 10)prior to siliciding, at least one aerogel or xerogel of ceramic typerefractory material is put into place within the pores of thethermostructural composite material.

This implementation differs from that of FIG. 2 in that after theoptional step 10 of cleaning the accessible surfaces of thethermostructural composite material, impregnation is performed with acomposition containing a solution of a compound that is a precursor of aceramic type refractory material (step 82).

The term ceramic type refractory material is used herein to mean inparticular a material of the carbide, nitride, boride, or oxide type.

The precursor may be an organometalloid compound or an organometalliccompound. Thus, with a ceramic material made of silicon, the precursormay be an organosilicon compound. For example, the organosiliconcompound can be polycarbosilane (PCS) which is a precursor of SiC. Thesolvent is selected as a function of the compound used. For PCS, it ispossible to select a solvent from xylene, toluene, heptane, and hexane,for example. In order to gel (polymerize) the PCS, a catalyst may beadded to the impregnation solution, e.g. borodiphenylsiloxane. Anotherusable precursor of SiC is polyvinylsilane (PVS) which can give a gel byreacting with a peroxide.

After impregnation with the composition containing a solution of ceramictype refractory material precursor, in situ gelling is performed withinthe pores of the composite material, which gelling is followed byripening (step 84).

Gelling is performed in a stove, in a closed enclosure at a temperaturebelow the evaporation temperature of the solvent.

After gelling and ripening, there follows a step of supercriticaldrying, possibly after solvent exchange (step 86) in order to obtain anaerogel of the precursor of the ceramic type material (step 88), or astep is performed of drying by evaporation (step 90) in order to obtaina xerogel of the precursor of the ceramic type material (step 92).

The steps of supercritical drying or drying by evaporation are similarto the steps 18 and 22 of the method of FIG. 2.

Pyrolysis (step 94) is then performed to obtain an aerogel or xerogel ofceramic type refractory material (step 96). Pyrolysis is performed at atemperature in the range 600° C. to 2000° C. (or even more) depending onthe nature of the precursor.

A plurality of aerogels and/or xerogels of the ceramic type materialprecursor can be formed in succession prior to pyrolysis.

It is also possible to form in succession an aerogel or xerogel ofcarbon precursor and then an aerogel or xerogel of ceramic type materialprecursor, or vice versa.

After pyrolysis, a siliciding step is performed (step 98) byimpregnating the composite material with a molten silicon type phase.Siliciding can be performed as described above.

This produces a thermostructural composite material part in which thematrix includes a silicon type phase holding captive at least oneaerogel or xerogel of ceramic type refractory material.

Depending on the nature of the aerogel or xerogel, siliciding may benon-rectional, leaving the aerogel or xerogel unchanged.

The method of treating thermostructural composite material parts inaccordance with the invention enables parts obtained by the method tohave conferred on them particular properties that are stable andreproducible in terms of thermal conductivity, mechanical strength,leakproofing, and tribological characteristics.

Thus, in a particular application of the method, it can be used toobtain electrodes of silicided thermostructural composite material, inparticular of silicided C/C composite material.

Compared with electrodes of C/C composite material as obtained withouttreatment by the method in accordance with the invention, it is thuspossible to improve mechanical strength and to make the electrodes moreleakproof, without affecting electrical conductivity.

Electrodes obtained by a method of the invention are suitable for use inparticular as anodes and/or cathodes and/or accelerator grids for plasmaengines or ion engines.

Another, similar application is making bipolar plates for fuel cells.

In another particular application, described below in the examples, amethod of the invention can be used to obtain friction parts for brakesor clutches having improved tribological properties.

A method in accordance with the present invention can also be used forbonding together thermostructural composite material parts. Such a bondcan be desired in particular for the purpose of obtaining an element ofcomplex shape or of dimensions that are relatively large and difficultor impossible as a single part.

Two parts 100, 102 of thermostructural composite material for assemblingtogether (FIG. 11) are united with respective surfaces placed side byside. In the figure, for simplification purposes, the parts shown aresimple in shape.

The parts 100 and 102 are impregnated in a solution having componentsthat serve, after gelling, ripening, and drying, to obtain an aerogel orxerogel made of a carbon precursor or a ceramic material precursor. Theaerogel or the xerogel is formed within the accessible pores of thecomposite material of the parts 100 and 102, and also in the interfaceor joint 104 defined by the adjacent surfaces of the parts.

After the precursor has been transformed by pyrolysis, siliciding isperformed with a silicon type phase.

This produces a silicon type phase containing a nanometric array ofrefractory material that extends continuously through the pores of theassembled-together parts and within the joint between them, thus bondingthe parts together.

The stages of forming an aerogel or xerogel made of a carbon precursoror a ceramic precursor (where such stages can be repeated), ofpyrolysis, and of siliciding, are all performed as described above.During siliciding, the parts 100 and 102 are fed by respective drains106 ₁ and 106 ₂ connecting the parts to different crucibles containingthe silicon type phase, or to a common crucible.

Example 1

The method was implemented on a block of SiC/SiC composite materialhaving fiber reinforcement made essentially of SiC and an SiC matrixobtained by chemical vapor infiltration. The fibers were fibers soldunder the name “Hi-Nicalon” by the Japanese supplier Nippon Carbon Co.,Ltd. The SiC matrix was obtained in well-known manner from a reactiongas comprising methyltrichlorosilane (MTS) and hydrogen gas (H₂).

The block of SiC/SiC material was impregnated with an aqueous solutioncontaining a mixture of phloroglucinol and formaldehyde together withsodium carbonate acting as a catalyst.

Impregnation was performed by immersing the block of SiC/SiC material ina bath of the solution within a vessel in an evaporated closedenclosure, and subsequently returning to atmospheric pressure.

Gelling and then ripening was performed by raising the temperature ofthe block of impregnated SiC/SiC material to about 55° C. for about 24hours (h).

The organic aerogel was then obtained by supercritical drying. Solventexchange was performed to replace the water with carbon dioxide (CO₂) byproceeding with an intermediate exchange with ethanol, as described inthe above-mentioned article by L. Kocon. Drying was performed byexceeding the critical point of CO₂ (31.1° C. and 7.3 megapascals(MPa)).

The resulting organic aerogel was transformed into a carbon aerogel bypyrolysis at 1000° C. under an inert atmosphere, e.g. under nitrogen orargon.

Siliciding was then performed by delivering molten silicon by means of adrain made up of a cord of “Hi-Nicalon” SiC fibers having one endimmersed in a crucible containing molten Si, and its other end incontact with the block of SiC/SiC material in which the pores werefilled with the carbon aerogel. Siliciding was performed with molten Siat about 1450° C.

Examination of the resulting material under a microscope (FIG. 12)showed that all of the carbon aerogel had been converted into SiC byreacting with the molten silicon, and that free silicon remained withinthe material. In FIG. 12, silicon carbide appears gray, while freesilicon appears white in a pore P of the composite material. There canalso be seen the fibers F of the composite material and the SiC matrixwhich surrounds and interconnects the fibers F.

FIG. 14 shows the same SiC/SiC thermostructural composite material aftersiliciding with molten silicon, but without previously introducing anyaerogel into the pore P of the composite material. It can be seen thatthe pore is merely filled with free silicon.

Furthermore, the thermal conductivity of the silicided SiC/SiC materialwas measured and found to be multiplied by 4 compared with the sameSiC/SiC material in which densification was followed by chemical vaporinfiltration.

Example 2

The method was implemented on a block of C/SiC material having carbonfiber reinforcement based on a precursor of preoxidizedpolyacrylonitrile (PAN) and having an SiC matrix obtained by chemicalvapor infiltration from a reaction gas containing a mixture of MTS andof H₂.

The block of C/SiC material having porosity of about 15% by volume wasimpregnated with an aqueous solution containing a mixture of resorcinoland formaldehyde, together with sodium carbonate.

Impregnation was performed by immersion in a bath under a vacuum.

Gelling and ripening were performed by raising the temperature of theblock of impregnated C/SiC material to about 55° C. in a stove for aperiod of about 24 h.

An organic xerogel was then obtained by controlled drying causing thewater to evaporate from the gel. For this purpose, the temperature wasraised progressively and slowly (2° C./h) up to about 90° C., which wasmaintained for about 5 h.

The resulting organic xerogel was transformed into a carbon xerogel bypyrolysis by progressively raising the temperature up to about 950° C.

Siliciding was then performed with a silicon type phase comprising 75%Si and 25% Fe (in atomic percentage).

Examination under a microscope (FIG. 13) of the resulting material showsthat the carbon xerogel had been converted into SiC. FIG. 13 shows thatthe xerogel introduced into the pore P in the material is cracked(fissure f), the fissure being filled with free silicon havinginclusions of iron silicate. As in FIG. 12, the fibers F and the SiCmatrix m of the composite material can be seen.

Example 3

The procedure was the same as in Example 1, except that an SiC/SiCmaterial was used with fiber reinforcement based on “Nicalon NLM 202”fibers of limited temperature stability and siliciding was performedwith a mixture of silicon and germanium in 50/50 atomic percentage.

The liquidus temperature of the Si+Ge phase was about 1250° C. Aftermelting the SiC+Ge mixture at 1280° C., the carbon aerogel wasreactively impregnated and transformed into carbide without degradingthe “Nicalon NLM 202” fibers.

Example 4

A block of SiC/SiC material was used as in Example 1, but impregnationwas performed with a composition containing a solution ofpolycarbosilane (PCS) in xylene in the presence of borodiphenylsiloxane.

Impregnation was performed by immersing the block of SiC/SiC materialunder a vacuum and then returning to atmospheric pressure.

Gelling was subsequently performed by raising the block of impregnatedSiC/SiC material to a temperature of about 80° C. in a sealed enclosure.

A xerogel of SiC precursor was obtained merely by evaporating thesolvent in a ventilated stove at 80° C.

An SiC xerogel was subsequently obtained by pyrolysis performed byraising the temperature progressively up to 900° C.

Molten silicon was used to fill in the pores of the SiC xerogel and wasintroduced by siliciding performed in the same manner as in Example 1.

Example 5

The method was implemented on a block of C/C composite material havingcarbon fiber reinforcement from a preoxidized PAN precursor and apyrolytic carbon (PyC) matrix obtained by chemical vapor infiltration.The densification of the PyC matrix was interrupted when the block ofC/C material had reached a specific gravity of about 1.4, correspondingto a residual porosity of about 27% by volume.

A carbon xerogel was put into place within the pores of the C/C materialblock in the manner described in Example 2, and was converted into anSiC xerogel by siliciding, likewise in the manner described in Example2.

This produced a C/C—(SiC—Si) material characterized in that themacropores in the initial C/C material were filled in not by a siliconphase but by a composite system constituted by a silicon matrixsubdivided and reinforced by an SiC xerogel. This subdivision andreinforcement of the silicided phase modified the tribologicalproperties of the initial C/C material, making it possible to obtain ahigh coefficient of friction and low wear.

Example 6

The procedure was the same as in Example 5, but two xerogels of carbonwere put into place in succession within the pores of the block of C/Cmaterial prior to siliciding.

The resulting C/C—(SiC—Si) material differed from that of Example 5 byhaving a greater SiC/Si volume ratio.

By varying the quantity the carbon xerogel, it was thus possible tomodify the tribological properties of the resulting material.

The C/C—(SiC—Si) materials obtained from Examples 5 and 6 are suitablefor use in particular in friction applications, specifically for makinghigh performance brakes for airplanes, land vehicles, clutches, . . . .

It is thus possible to make friction parts in the form of airplane brakedisks with such C/C—(SiC—Si) composite materials.

A set (or heat sink) of rotor disks and of stator disks for an airplanebrake can be made up of such disks.

In a variant, in a set of rotor disks and stator disks for an airplanebrake, some of the disks, e.g. the rotor disks (or the stator disks) canbe made of C/C—(SiC—Si) composite material while the other disks, i.e.the stator disks (or the rotor disks) are made of non-silicided C/Ccomposite material.

Example 7

A test piece of C/SiC composite material was made having dimensions of50 millimeters (mm)×28 mm×5 mm and was treated as in Example 2.

After siliciding, a helium leak test was performed on the resulting testpiece of C/SiC—(SiC—Si) material. The leakage value measured was 10⁻⁴pascal cubic meters per second (Pa·m³/s) which is a low value. Prior totreatment, such a measurement could not be performed at all, because ofthe high permeability of the material.

This example shows the capacity of the method of the invention to makethermostructural composite materials leakproof in bulk.

It should be observed that leakproofing can be further improved byforming a coating of ceramic material, e.g. of SiC, on the surface ofthe silicided composite material. Such a coating can be obtained bychemical vapor deposition or infiltration.

1. A silicided thermostructural composite material part, characterizedin that the silicided thermostructural composite material includes asilicon type phase containing at least one nanometric array of ceramictype refractory material.
 2. A part according to claim 1, characterizedin that the nanometric array is made of silicon carbide.
 3. A partaccording to claim 1, characterized in that the silicon type phase isconstituted by silicon and/or germanium.
 4. A friction part according toclaim 1, characterized in that it comprises a carbon/carbon compositematerial having its pores filled at least in part with a silicon phasecontaining a nanometric array of silicon carbide.
 5. A set of rotor andstator disks for an airplane brake, characterized in that the disks arefriction parts according to claim
 4. 6. A set of rotor and stator disksfor an airplane brake, characterized in that the rotor disks—or statordisks—are friction parts according to claim 4, while the other disks aremade of a non-silicided carbon/carbon composite material.
 7. A partaccording to claim 2, characterized in that the silicon type phase isconstituted by silicon and/or germanium.
 8. A friction part according toclaim 7, characterized in that it comprises a carbon/carbon compositematerial having its pores filled at least in part with a silicon phasecontaining a nanometric array of silicon carbide.
 9. A set of rotor andstator disks for an airplane brake, characterized in that the disks arefriction parts according to claim
 8. 10. A set of rotor and stator disksfor an airplane brake, characterized in that the rotor disks—or statordisks—are friction parts according to claim 8, while the other disks aremade of a non-silicided carbon/carbon composite material.